Can I use a class attribute as a default value for an instance method?

Question

I would like to use a class attribute as a default value for one of the arguments to my class's __init__ method. This construct raises a NameError exception, though, and I don't understand why:

class MyClass():     __DefaultName = 'DefaultName'     def __init__(self, name = MyClass.__DefaultName):         self.name = name 

Why does this fail, and is there a way to do this that works?

Answer 1

That's because, according to the documentation:

Default parameter values are evaluated when the function definition is executed. This means that the expression is evaluated once, when the function is defined

When __init__() is defined, the definition of MyClass is incomplete, as it's still being parsed, so you can't refer to MyClass.__DefaultName yet. One way to work around that is to pass a special unique value to __init__(), such as None:

def __init__(self, name=None):     if name is None:         name = MyClass.__DefaultName     # ...